We shall continue our research program on B12-dependent methyl-transfer to a selected group of toxic elements. We have identified six alternate mechanisms which lead to cleavage of the Co-C bond of methylcobalamin to generate either a carbanion (CH3 minus), a radical (CH3.), or a carbonium ion (CH3 plus), as the leaving group. From our studies with HgII, PbIV, T1III, PtIV, SeVI, AsV, SnII, RSH, CrII, IrIV, and AuIII there appears to be a correlation between standard reduction potential (E degrees) and the selected mechanism for methyl-transfer. We intend to explore this relationship further with different salts of the above metals and metalloids. Our work on the demethylation of methylcobalamin by platinum salts has opened up an exciting new area which may have implications relevant to the mechanism of action of the platinum anti-tumor drugs. In addition, we intend to continue our work on mechanisms for the toxicity of metal-alkyls, as well as our study on electron transfer, and regulation, of B12-dependent ribonucleotide reductase.